This invention relates to the removal of gases from liquids, particularly complex liquids such as pulp slurries.
It is well known in the papermaking industry that the presence of gases in the pulp stock gives rise to a variety of problems; for example the presence of gas bubbles in the papermachine headbox causes foaming of the pulp, a very undesirable condition. The amount of gas in the pulp stock varies from about 3.1% to about 8.8% by volume under normal operating conditions. The gases exist in the pulp stock in three states, namely, as bubbles, as gas in solution, and as gas adsorbed on fibers. The percentages of gases existing in those states are interchangeable depending on the location in the system between the pulp pump and the papermachine headbox. The amount of gases in the bubble state varies from about 37% to about 67% of the total amount of gas. Hydrocyclone stock cleaners supply a large part of the gases to the pulp stock, the air core in the cleaner, with its very low pressure, supplying gases into the accept line with about 85% of these gases being in the bubble state. Part of this gas, downstream of the hydrocyclone cleaners, will change back into the other states, as recited above, if action is not taken to remove such gases. It is therefore of importance to provide suitable means between the hydrocyclone cleaning system and the headbox for removal of these gases while a large portion of same are still in the bubble state.
The prior art has provided a number of devices and processes for removal of gaseous phases from a liquid phase. A first group of these devices is capable of removing gases from what might be termed "clean" liquids, i.e. liquids wherein there is little or no suspended material. The second group of devices is concerned with the removal of gases from complex fluids such as suspensions of cellulose fibers in water, i.e. pulp stock for use in paper manufacture.
One early form of deaerator is shown in U.S. Pat. No. 580,169 dated Apr. 6, 1897 to Washington. This deaerator includes lateral vanes at the top of a tubular member and ridges adjacent the bottom of it that, together with a group of pipes and fingers positioned transverse to the flow direction, obstruct the flow, creating increased turbulence and eddies in the fluid. Gases are collected in a tank at the exit end of the deaerator by means of a system of pipes in communication with a number of different gas collecting points in the apparatus. This deaerator is adapted for use with "clean" liquids only; the numerous dead "pockets" in the various flow obstructing devices render it unsuitable for use with pulp slurries as the fibers would immediately accumulate on or in the various obstructions thus causing the apparatus to eventually plug and become inoperative.
Another device for removing gases from liquids is shown in U.S. Pat. No. 3,525,196 issued Aug. 25, 1970 to Brieskorn. This system is described as being useful for extracting gases from the cooling system of an internal combustion engine. The apparatus includes a tubular housing having inlet and outlet openings of about the same diameter as the flow lines connected thereto and surrounded by a housing of larger diameter. The inner housing is provided with an opening in the region of the upper flow path upstream and another opening near the lower flow path downstream. Vent gases are exhausted by an upper opening of the larger diameter housing which is connected with an expansion chamber provided with a pressure relief valve. The liquid flow is moved through the inner housing along the openings thereby allowing air or gas to escape by the upstream opening and discharged fluid to re-enter by the downstream opening of the inner housing. Again, as with the Washington patent, the apparatus is useful for generally "clean" liquids but not for complex liquids such as pulp slurries. Furthermore, the fluid velocities through the device must be kept relatively slow as compared with those velocities which are commonly used in conjunction with pulp slurries.
Another group of devices works on the principle of passing a very slowly flowing bi-phase mixture through a big tank and exposing its thin layer fluid surface to allow escape of gases therefrom. The gaseous phase is caused to pass through a series of baffles which provide sufficient surface area to allow for separation and collecting of liquid droplets from the gaseous phase. Tanks or vessels are generally not pressurized and empty themselves by means of fluid head energy.
A still further group of deaerating devices, related to paper pulp stock cleaning, involve means for atomizing the stock by spraying while applying a high vacuum thereto. High deaerating efficiencies are achieved but these types of installations tend to be relatively complex and expensive.